The purpose of an automobile transmission is to provide a neutral, at least one reverse and one or more forward driving ranges that impart torque from the engine to the driving wheels, as required for the necessary driving force and the desired performance.
As is well known to the art, one widely accepted form of an automatic, vehicular transmission employs a compound planetary gear set that utilizes two braking bands to apply the friction required to obtain the desired function of the compound planetary gear set. The operator selects the driving range from the neutral, forward (either the standard drive, the "Intermediate" or the "Lo" forward selections) or reverse, and the transmission automatically changes gear ratios in relation to the vehicle speed and the engine torque input, as permitted within the range selected. Vehicle speed and engine torque signals are constantly fed to the transmission in a manner well known to the art in order to provide the proper gear ratio for maximum efficiency and performance at all throttle openings.
A planetary gear train consists of a center, or sun, gear, an internal gear and a planetary carrier assembly which includes and supports the smaller planet gears, or pinions. When the sun gear is held stationary and power is applied to the internal gear, the planetary gears rotate in response to the power applied to the internal gear and thus "walk" circumferentially about the fixed sun gear to effect rotation of the carrier assembly in the same direction as the direction in which the internal gear is being rotated.
When any two members of the planetary gear train rotate in the same direction and at the same speed, the third member is forced to turn at the same speed. For example, when the sun gear and the internal gear rotate in the same direction, and at the same speed, the planet gears do not rotate about their own axes but rather act as wedges to lock the entire unit together to effect what is known as direct drive.
Whenever the carrier assembly is restrained from spinning freely, and power is applied to either the sun gear or the internal gear, the planet gears act as idlers. In that way the driven member is rotated in the opposite direction as the drive member. Thus, when the reverse drive range is selected, a band assembly is actuated frictionally to engage the carrier assembly, and restrain it against rotation, so that torque applied to the sun gear will turn the internal gear in the opposite direction in order to reverse the rotational direction of the drive wheels, and thereby reverse the direction of the vehicle itself. The present invention relates to the servo mechanism employed to actuate the friction band assembly.
It should be appreciated that a second friction applying band assembly may also be employed when the engine compression, acting through the transmission, is being employed to effect a braking action. To understand this operation it is desirable to know that in a compound planetary gear set, multiple planetary gear sets may be employed, and adjacent planetary gear sets may utilize sun gears fabricated in one piece. Such adjacent planetary gear sets also generally connect the carrier of the first set to the internal gear of the second set. To make the two planetary gear sets effective a roller clutch assembly is generally employed to hold the carrier of the second set against rotation in at least one direction. A sprag assembly is also generally employed selectively to preclude the common sun gears from rotating in one selected direction.
When the gear selector is positioned in the Intermediate range to maintain the gear ratio in a range which will not exceed, for example, what would be considered a "second" gear, and the accelerator is released, the vehicle will decelerate using the engine compression as the braking force. In this situation the drive wheels are not being driven by the output shaft of the transmission but, conversely, the drive wheels are driving the transmission through the output shaft. With the power thus being applied to the output shaft by the drive wheels, there is a tendency for the sprag within the transmission to overrun, and thereby become ineffective. To preclude that result, and thereby to prevent the sun gear from over-running, a second friction band assembly may be employed to keep the transmission in "second" gear in order to assure effective, dynamic "braking" by virtue of the engine compression. A servo mechanism of the type embodying the concepts of the present invention may also be employed to actuate this second band assembly.
For even greater engine braking the transmission can be placed in the "Lo" range. At speeds below approximately 40 MPH the transmission will shift to first gear. When the vehicle is coasting in first gear the roller clutch tends to over-run, and in this situation the first described friction band assembly may be employed to restrain the conjoined carrier and internal gear of the adjacent sets from over-running the roller clutch assembly, and thus maintain the transmission in first gear in order to assure effective, dynamic braking by virtue of the engine compression.
The servo mechanism employed to operate each band assembly generally incorporates a hydraulically actuated piston assembly. Historically, the configuration of the transmission case within which the planetary gear set is housed was further complicated because it included a boss which had to be accurately machined in order to provide the necessary piston chamber within which an actuating piston assembly could reciprocate to effect operation of an associated friction band assembly.
Transmission cases have historically been, and continue to be, metallic castings. Castings do, on occasion, incorporate voids, but even microscopic voids, which are likely to be considered as determining the porosity of the casting, can be adversely located, and can be of such localized abundance, that when the casting is machined one or more of the machined surfaces will prove to be unacceptable for the intended purpose of those surfaces. As should be readily apparent, the structural properties of strength and hardness required to make an acceptable transmission case are not necessarily conducive to providing a readily machinable casting. In fact, variations in the section thickness of a casting, and particularly a casting having the complexity of a transmission case, can cause localized hard, or soft, spots. Although one might ideally desire a more homogenous casting, such localized variations in the physical properties of the casting may not themselves negate the suitability thereof to serve as a transmission case. Unfortunately, however, such localized variations can adversely affect the ability of the casting to be acceptably machined.
For example, the reaction of a machine tool against a localized hard spot can cause microscopic, if not macroscopic, grooves which could well preclude the effective sealing of the pressure chamber required for the piston assembly in a servo mechanism built into the transmission case, and such flaws might not be identified until after at least partial assembly of the transmission. But even if any such flaw were detected at the earliest possible opportunity, the result would be a rejected transmission case.
In today's highly automated production facilities the rejection of components, and particularly major components, can not only severely increase the cycle time required to deliver that component to the assembly line but can also significantly increase the cost of that particular component.